Determination of OATP Mediated Drug-Drug Interaction Potential in Pooled Cryopreserved Human Hepatocytes Leena Laitinen, Pamela McCarthy, Brian Boucher and Niresh Hariparsad Early identification and risk assessment for the potential for drug-drug interactions (DDIs) has become an integral component of the drug discovery process. Since the liver is the main drug eliminating organ, it is of great importance to have preclinical tools which accurately predict DDIs. In certain instances, even though drugs have poor permeability into the liver due to their physicochemical properties, they are efficiently taken up into the liver and excreted into bile. Recent evidence suggests that uptake transporters such as organic anion-transporting polypeptides (OATPs) are involved in facilitating this process. Among these transporters, OATP1B1 and OATP1B3 are specifically expressed in the liver and have broad substrate specificities. Both of these transporters are known to be involved in DDIs in the clinic. When rifampicin was co-administered with bosentan, a 500% increase in the AUC of bosentan was observed. Consequently, it is important in the early drug development process to accurately predict the potential for DDIs associated with these transporters. While cell lines overexpressing these transporters form a key component of assessing the DDI potential, human hepatocytes have a major advantage, since they have the full complement of phase I and phase II drug metabolizing enzymes and transporters. As such, we conducted validation studies with a five-donor pool of cryopreserved human hepatocytes. For this validation, we chose three compounds (cyclosporine, rifampicin and lopinavir) known to cause DDIs in the clinic when coadministered with statins and three compounds (amprenvir, ritonavir and gemfibrozil) that do not cause DDIs in the clinic when coadministered with statins. We used pitavastatin as the probe substrate for all our inhibition studies. Using the R-value approach (Hirano et al., (2006) DMD 34, 1229-1236) we were able to qualitatively predict the DDI potential in the clinic. Having established the utility of the R-value approach, we chose two protease inhibitors used in the treatment of Hepatitis-C. Since the general approach to the treatment of HCV involves administering a cocktail of drugs to patients, there is an increased chance of DDIs. Our studies suggest that both HCV compounds are actively taken up into hepatocytes and therefore could be victims of DDIs if coadministered with compounds such as rifampicin or cyclosporine which are known to be potent OATP inhibitors. In our studies, compound A was an inhibitor of OATP1B with an IC50 = 9.6 ▒ 0.4 ÁM, while compound B had an IC50 value of approximately 50 ÁM. Additional studies are currently under way in our lab to further assess the DDI potential of these compounds both as perpetrators and victims. Overall, our studies suggest that cryopreserved human hepatocytes can be used as a preclinical tool to identify DDI potential of compounds during the drug development process.